<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <!-- 类选择器 -->
  <style>
   .textclass{
     color: red;
   }
   
div span {
  color: purple;
}
  </style>
</head>

  <p>段落一</p>
  <p class="textclass">段落二</p>
  <p>段落三</p>
  <button class="textclass">按钮</button>
  
  <span> span 标签</span>
  
  <div>
    <span>这是 div 的儿子 span</span >
  </div>
</body>
</html>